Fix mpi gather scatter
This MR fixes the wrong computation of the data length in igather and iscatter.
In addition, it adds a test that uses igather and iscatter with a datatype that has length larger than 1.
This MR fixes the wrong computation of the data length in igather and iscatter.
In addition, it adds a test that uses igather and iscatter with a datatype that has length larger than 1.